For decorative articles of a wristwatch (hereinafter referred to also as “wristwatch parts”), such as a wristwatch case, and so forth, heavy use has thus far been made of titanium (hereinafter “titanium” includes pure titanium, and titanium alloy) and stainless steel as the constituent material thereof. In particular, a share of titanium for use in the wristwatch parts has lately increased because of its sturdiness, excellent corrosion resistance, light weight, and good biocompatibility causing no allergic reaction to metal to occur to a human body. Further, titanium has been in widespread use not only for wristwatches but also in various industrial sectors in order to utilize its excellent characteristics such as light weight, high strength, and excellent corrosion resistance.
A pure titanium crystal is hexagonal close-packed structure at room temperature, but turns into a body-centered cubic structure at a temperature not lower than 882° C., that is, the transformation point thereof (temperature at which transformation occurs). Accordingly, if this temperature (that is, 882° C.) is exceeded in the process of manufacturing a wristwatch part, a surface condition of the wristwatch part undergoes a change following transformation of its crystals, so that re-polishing and so forth needs to be performed. Because of a risk of an increase in the number of process steps to that extent, it is regarded desirable to treat titanium at a temperature not higher than the transformation point thereof.
Meanwhile, as stainless steel is moderately weighty, and excellent in corrosion resistance, it is most widely used as the constituent material of wristwatch parts. Various kinds of stainless steels are available, but as the constituent material of the wristwatch parts, much use is being made of an austenitic stainless steel such as SUS 304, SUS 316L, and so forth from the viewpoint of corrosion resistance.
Now, a conventional wristwatch case as a wristwatch part is generally manufactured by forming a wristwatch case body integrally with an end-piece (the end-piece is a part for linking a watch band with the wristwatch case body). In such a case, it is difficult to manufacture parts of a complex design by a forging method in comparison with a case of using a casting method, however, it has lately become a general practice to manufacture the wristwatch parts by the forging method such as hot forging, cold forging, and so forth.
Further, with reference to the wristwatch parts, there has lately increased a demand from a design point of view for providing diverse variation in surface finishing thereof by applying surface processing such as mirror finishing, hair-line finishing, honing, and so forth on a portion-by-portion basis. Herein, the mirror finishing means shining mirror finish by polishing, and the hair-line finishing means surface finishing with minute hair-line-like directional lines. The honing means surface finishing with fine asperities, avoiding use of plating, and is also called mat. For example, in the case of a wristwatch case, there is a demand for application of combined surface finishing by applying the honing to a wristwatch case body, and the mirror finishing to an end-piece to thereby vary surface finishing by portions of the wristwatch case. Further, as to the wristwatch parts, there is also a demand for exhibiting diverse variation such as color variation, pattern variation, and so forth by applying a film-depositing process such as plating, IP (ion plating), or coating besides applying the surface processing.
However, the wristwatch parts each are small in whole size, so that, for example, in the case of a wristwatch case, if the wristwatch case is manufactured by integrating an end-piece with a wristwatch case body at the outset, it will become extremely difficult to finish up the surface thereof beautifully (by exhibiting variation in surface finishing) by applying thereafter the surface processing and film-depositing process, varying by portions. There is also a case where an end-piece of a wristwatch case is desired to exhibit a design feature by forming the end-piece in a complex shape, and in such a case as well, if the wristwatch case is manufactured by integrating the end-piece with the wristwatch case body at the outset, it will become difficult to work on the end-piece later.
Accordingly, in case there is a strong demand from a design point of view for manufacturing the wristwatch case with variation in the surface finishing or for providing the wristwatch case with a design feature by forming the end-piece into a complex shape, the wristwatch case body and the end-piece are separately formed, and processing as desired is applied thereto, respectively, to be thereby integrated with each other by engaging one with the other or connecting together. By so doing, processing as desired can be applied to respective parts of the wristwatch case, so that the wristwatch case is provided with design variation in abundance.
In the case of bringing the end-piece into engagement with the wristwatch case body, there is available a method whereby a pin is pressed inbetween the wristwatch case body and the end-piece, and both parts are engaged with each other through the intermediary of the pin. This method, however, has a drawback in that it is difficult to maintain a condition of engagement of the wristwatch case body with the end-piece over an extended period of time, resulting in low reliability in respect of durability. Furthermore, with the method, it is difficult to render the pin used for engaging both the parts with each other to be invisible from outside, so that the method has another drawback in that there is the risk of a design feature of a wristwatch case being detracted due to visibility of the pin from outside.
Further, for connecting the end-piece with the wristwatch case body, methods of brazing using a brazing filler metal and welding are available. Brazing has a problem of brazing filler metal flow, however, with this method, surface finishing even after connecting the wristwatch case body with the end-piece can be relatively excellent. However, brazing has a shortcoming in respect of corrosion resistance at connection portions. As described above, in the case of using titanium as the constituent material of the wristwatch case body and the end-piece, when attempting connection of both the parts at a temperature not higher than the transformation point of titanium, it is not possible to find a brazing filler metal that is fully suitable for the purpose. Above all, silver solder (for example, JIS: BAg—8, and so forth) is appropriate. However, since the silver solder has very poor corrosion resistance, there is the risk of discoloration, rust, etc. occurring to connected parts with the passage of long time after connection, thereby considerably impairing visual quality of the wristwatch case.
Meanwhile, some wristwatch parts require high waterproof. However, particularly with the wristwatch case, it is difficult to maintain waterproof on a long term basis upon connection of the wristwatch case body with a crown pipe, so that there exists a problem unavoidable from a structural point of view in that reliability in respect of waterproof is prone to deteriorate.
In the case of the wristwatch parts, respective parts can be also connected with each other by welding besides brazing. If connected by welding, however, burn-like marks are left out, thereby impairing visual decorativeness. Among others, projection welding is available for melting minute juts called projections, but the applicable parts are limited because parts to be welded undergo, for example, a change in size (shape) upon welding, thereby imposing restriction on the wristwatch parts from a design point of view.
Thus, the conventional wristwatch parts used to have various problems in the case where the respective parts (for example, the wristwatch case body, the end-piece, crown pipe, etc.) are formed individually, surface finishing as desired is applied thereto, respectively, and those parts are subsequently integrated with each other by engagement or connection with each other. That is, in such a case, it has been extremely difficult to provide the wristwatch parts with not only sturdiness sufficient to maintain for long duration a condition in which the respective parts are integrated with each other by engagement or connection but also corrosion resistance as well as waterproof while keeping visual decorativeness in excellent state.
The invention has been developed in order to resolve such problems as described, encountered by the conventional wristwatch parts, and it is an aspect of the invention to provide a decorative article of a wristwatch, made up of two or more parts such as a wristwatch case made up of a wristwatch case body and an end-piece, and a method of manufacturing the same, wherein there are provided not only sturdiness sufficient to be able to maintain a condition of engagement or connection of the respective parts with each other for long duration but also corrosion resistance as well as waterproof while keeping visual decorativeness in excellent state, and having design variation in abundance.